In the assembly of components of an orthodontic appliance, e.g., as part of a system commonly referred t o as "races," brazing is often used to bond the components together. Brazing is intended to provide bonds having great strength, temperature resistance, and corrosion resistance.
Brazing refers t o a process of joining metal components, often of dissimilar composition, to each other. Typically, a brazing alloy that has a melting point lower than the melting point of the components to be joined is interposed between the components to form an assembly. The assembly is then heated to a temperature sufficient to melt the brazing, alloy but below the melting point of the components to be joined. The brazing, alloy melts and wets the joint between the components, often by capillary flow of the melted brazing alloy. In some instance, a certain degree of metallurgical reaction might also occur between the brazing alloy and the base metal of the components. Once the joint is wetted, the assembly is cooled so that the brazing alloy solidifies, thus forming a strong bond between the components.
Brazing alloys used in joining, stainless steel components in the orthodontic and dental fields, categorized by the element of highest weight percentage, have been silver-based, gold-based, or nickel-based. In orthodontic and dental applications, the selection of a brazing alloy class impacts the visual aesthetics (e.g,., the color match between the resulting joint and the components), strength, corrosion resistance, and tarnish resistance of the resulting brazed appliance.
Silver-based brazing, alloys are typically alloyed, individually or in combination, with copper, tin, or zinc in order to lower the melting, point of the brazing, alloy. In brazing, stainless steel, silver-based brazing, alloys have the advantage of providing a low melting temperature and good bond strength. However, the color of the brazed joint resulting from the use of silver-based brazing, alloys does not match stainless steel because the brazed joint tarnishes to a dark color over time. Additionally, data suggest that using silver-based brazing alloys that do not include nickel can result in interface corrosion between the base metal of the components and the braze fillet due to nickel depletion at the braze fillet/base metal interface. See, e.g., I. Kawakatsu, Welding J., June (1973) pp. 233-239.
Gold-based brazing alloys are typically alloyed, individually or in combination, with nickel, copper, palladium, and silver. Gold-based brazing, alloys arc often used with stainless steel because gold-based brazing alloys have good strength, ductility, and tarnish resistance. However, the yellow color of gold-based brazing, alloys contrasts with the stainless steel used in the components to be brazed. Moreover, gold is galvanically noble to the active stainless steel along the braze fillet/base metal interface. The galvanic action results in interface corrosion in laboratory tests in physiological saline (Ringer's solution).
Nickel-based brazing, alloys offer an excellent metallic color match to the stainless steel in orthodontic and dental appliances. Conventional nickel-based brazing, alloys are most commonly alloyed with boron in order to lower the melting, point of the brazing, alloy. Chromium also is often present to increase the ductility of the braze fillet while increasing the resistance to high temperature and corrosion. However, under manufacturing or use conditions, the braze joint formed from conventional nickel-based brazing, alloys is preferentially corroded.
What is needed, therefore, is a brazing alloy that has a color that is compatible with the stainless steel used in orthodontic and dental appliances, can form a strong, bond, and is corrosion and tarnish resistant.